1. Technical Field
The present invention relates to a temperature-sensor circuit that can operate with a particularly low voltage and is superior in voltage sensitivity with respect to a temperature, and a temperature compensated piezoelectric oscillator using the same.
2. Related Art
In recent years, a piezoelectric oscillator is used in many fields from communication device such as cellular phones to commercial-off-the-shelf devices such as a quartz-crystal clock due to its high frequency stability, compact size and lightweight, low cost, and the like. In particular, a temperature compensated piezoelectric oscillator (TCXO) compensating a frequency temperature characteristic of a piezoelectric resonator is widely used in cellular phones and the like that require frequency stability.
JP-A-6-276020 discloses a temperature compensated crystal oscillator provided with a voltage generating circuit generating a desired voltage based on a temperature sensor. The temperature sensor is, as shown in FIG. 9A, configured so that one of end portions of a circuit including a plurality of diodes D1, D2, D3, . . . , Dn coupled in series is grounded and the other one of the end portions is coupled with a power supply Vcc through a resistor R51. A junction joining an anode of the diode D1 and the resistor R51 in the temperature sensor is defined as an output OUT so as to obtain an output voltage from there. This makes use of variation of a potential difference at a junction of diodes with respect to a temperature change. That is, when a direct-current voltage is applied to one end of the resistor R51, a voltage variation corresponding to the temperature change can be taken from the output OUT. As shown in FIG. 9A, if n number of diodes are coupled in series, sensitivity of the output voltage with respect to the temperature becomes n times more than that of one diode, providing a voltage nearly proportional to the temperature as shown in FIG. 9B.
Further, as shown in FIG. 9C, a temperature sensor in which a terminal of a resistor R53 in a series circuit coupling a thermistor TH and the resistor R53 is grounded, and the other terminal of the thermistor TH is coupled with a power supply Vcc through a resistor R52 is disclosed. This makes use of variation of a resistance value of the thermistor with respect to a temperature. A junction joining the thermistor TH and the resistor R52 is defined as an output OUT so as to obtain an output voltage from there.
However, in the temperature sensor using diodes as a temperature-sensitive element as shown in FIG. 9A, the diodes are coupled in a multistage manner, thereby increasing a power supply voltage Vcc. Therefore, the temperature sensor is not suitable for satisfying a recent requirement for a low voltage. Further, the temperature sensor using the thermistor TH as a temperature-sensitive element as shown in FIG. 9C has a problem in that linearity of the output voltage with respect to the temperature is unfavorable. Further, as shown in FIG. 9D, the temperature sensor using one diode as a temperature-sensitive element and amplifying an output voltage with an amplifier has a problem in that a noise generating from the amplifier affects an output of a temperature compensated piezoelectric oscillator.